Highlights from Recent Cancer Literature
Journal: Cancer Research (2012)

Fan ZC, Yan J, Liu GD, Tan XY, Weng XF, Wu WZ, et al. Real-time monitoring of rare circulating hepatocellular carcinoma cells in an orthotopic model by in vivo flow cytometry assesses resection on metastasis. Cancer Res 2012;72:2683–91.Hepatocellular carcinoma (HCC) is a highly metastatic disease in which circulating tumor cells (CTC) play a prominent role in promoting rapid metastasis. Monitoring the distribution and migration of these CTCs in the circulation at periodic intervals could be a feasible way to monitor disease progression, and this approach also has promise for rapid and precise evaluation of therapeutic strategies. In this study, Fan and colleagues developed a novel real-time in vivo flow cytometry–based tool for monitoring and measuring CTCs in both subcutaneous and orthotopic models of HCC. A human HCC cell line, HCCLM3, tagged with GFP, was used for proof-of-principle in this study. Remarkably, the sensitivity of the in vivo flow cytometry method for detecting CTCs was almost 2-fold higher than that of conventional whole-blood analysis. The CTC dynamics were more pronounced in the orthotopic model compared with the subcutaneous model. Moreover, a curative and debulking surgery of the orthotopic liver tumors significantly reduced the number of CTCs and subsequent lung metastases. This novel method, combined with relevant model systems, could be useful for surveillance of CTCs in highly aggressive tumors and with further development and refinement might one day provide an approach for monitoring metastasis and defining therapeutic efficacy in disease management.Nakasone ES, Askautrud HA, Kees T, Park JH, Plaks V, Ewald AJ, et al. Imaging tumor-stroma interactions during chemotherapy reveals contributions of the microenvironment to resistance. Cancer Cell 2012;21:488–503.Cancer chemotherapy is destructive and sometimes curative, but we know surprisingly little about what happens inside the actual tumor during treatment. Nakasone and colleagues offer a rare and surprising insight into drug distribution, cell death, and the role of the tumor stroma during chemotherapy of mouse mammary cancers. Early-, intermediate-, and late-stage mammary carcinomas were monitored in live mice with the use of spinning disk confocal microscopy. Cell death, primarily necrotic, was seen in both malignant and stromal cells 24 to 30 hours after doxorubicin chemotherapy, with the best response seen in intermediate-sized rather than early- or late-stage tumors. This difference was not due to cancer cell–intrinsic properties but rather was caused by the extent of vascular leakage into the tumor microenvironment. Necrotic cell death was a powerful stimulus for acute myeloid cell infiltration, but these cells, recruited by chemokines such as CCL-2, did not aid the response, and indeed it appeared that they contributed to resistance. CCL-2 binds to the chemokine receptor CCR-2 on myeloid cells, and response to doxorubicin and cisplatin was improved in breast tumors of CCR-2–null mice. Myeloid cells produce the matrix metalloprotease MMP-9, and vascular permeability and response to chemotherapy were also increased in MMP-9–null mice. Hence, at least in this mouse genetic model of breast cancer, the tumor microenvironment plays a critical role in response to chemotherapy via regulation of vascular permeability and innate immune cell infiltration. Addition of chemokine receptor antagonists or matrix metalloprotease inhibitors may increase the effectiveness and toxicity profile of traditional chemotherapy. However, interactions between myeloid cells and tumor vasculature are very complex; the timing and order of such combinations could be critical. The authors also elegantly demonstrate that intravital microscopy is a powerful tool for studying the tumor microenvironment at a cellular level—and for generating some inspirational cell movies.Menzies AM, Haydu LE, Visintin L, Carlino MS, Howle JR, Thompson JF, et al. Distinguishing clinicopathologic feature of patients with V600E and V600K BRAF-mutant melanoma. Clin Cancer Res 2012;18:3242–49.BRAF is mutated in approximately 50% of melanoma cases, with the vast majority of mutations at codon 600. Early reports suggested that 95% of the codon 600 mutations were V600E (valine to glutamic acid). Results from subsequent studies suggest that in certain geographic areas (Australia and the southern portion of the United States), the V600K (valine to lysine) mutation may occur in up to 20% of BRAF-mutant melanomas. Menzies and colleagues examine the clinicopathologic differences between V600E- and V600K-mutant melanoma. Melanomas that show the V600 mutation are responsive to BRAF inhibitors (vemurafenib and dabrafenib), prompting the authors to examine the relative efficacy of BRAF inhibitors in the 2 major V600 mutations in a cohort of patients from Australia. They found that younger patients with metastatic melanoma had a high rate of BRAF mutation, and when it occurred, there was a strong predominance of the V600E genotype. Older patients had a lower prevalence of BRAF mutations and a higher proportion of non-V600E mutations (mainly V600K). The authors speculate that long-term effects of UV irradiation may be associated with the increased V600K incidence in older patients. They examine the effect of BRAF inhibitors and find that these are efficacious in non-V600E mutant tumors. Given that a commonly used clinical test does not reliably detect non-V600E mutations, they caution that use of this test, particularly in older populations, may prevent a substantial number of patients from receiving a potentially efficacious drug.Hsieh AC, Liu Y, Edlind MP, Ingolia NT, Janes MR, Sher A, et al. The translational landscape of mTOR signalling steers cancer initiation and metastasis. Nature 2012;485:55–61.Thoreen CC, Chantranupong L, Keys HR, Wang T, Gray NS, Sabatini DM. A unifying model for mTORC1-mediated regulation of mRNA translation. Nature 2012;485:109–13.The mammalian target of rapamycin (mTOR) kinase is a key regulator of protein synthesis, with deregulated mTOR signaling associated with cancer. However, the full repertoire of translational targets downstream of mTOR has yet to be identified, and mixed results have been observed in cancer patients treated with mTOR inhibitors. Is the limited therapeutic benefit of clinical allosteric mTOR inhibitors, such as rapamycin, due to the inadequate blockade of mTOR activity? Two groups of researchers, Hsieh and colleagues and Thoreen and colleagues, identified mTOR translational targets at genome-wide resolution by using mTOR active site inhibitors that fully block mTOR signaling, along with a novel technology known as ribosome profiling. This cutting-edge approach involves deep sequencing of ribosome-protected mRNA fragments to yield a “snapshot” of actively translated mRNAs at single codon resolution. Hsieh and colleagues used this technique to elucidate the translational landscape of the prostate cancer genome downstream of mTOR hyperactivation. Notably, mTOR was found deregulated in nearly all cases of advanced prostate cancer. Thoreen and colleagues used transcriptome-scale profiling in immortalized mouse embryonic fibroblasts to determine the translational network in a noncancer setting. Both groups of investigators report novel cis-regulatory elements within the 5′UTR of mRNA translational targets of mTOR. Specifically, Hsieh and colleagues identified a pyrimidine-rich translational element (PRTE) sequence, and both groups showed that mRNAs containing the well-known 5′ terminal oligopyrimidine (TOP) motif are present in a subset of transcripts regulated by mTOR. Both groups also showed that mTOR-specific sensitivity conferred to transcripts containing PRTE, TOP, and TOP-like sequences was dependent on the activity of the eukaryotic initiation factor 4E (eIF4E) binding protein tumor suppressors (4EBP). In a hypophosphorylated state, 4EBPs bind to the oncogene eIF4E, preventing formation of the translation initiation complex on the 5′UTR of mRNAs. Hsieh and colleagues determined the functional relevance of a group of mRNAs controlled translationally by mTOR. This group of mRNAs harbors a PRTE sequence encoding a novel signature (YB1, MTA-1, CD44, and vimentin) that directs invasive features of human prostate cancer cells. Hsieh and colleagues demonstrated in vivo relevance by showing that mTOR regulates the expression of these 4 genes at the translational level in epithelial cells of the murine prostate gland. Loss of the PTEN tumor suppressor, a negative regulator of mTOR, led to increased translation of these invasive genes prior to metastasis formation. Significantly, this mTOR-dependent increase in the translation of these invasive mRNAs is druggable in vivo with the INK128 mTOR ATP site inhibitor, currently in clinical trials, which normalized expression of the invasive signature in PTEN-null epithelial cells. Importantly, INK128 prevented metastasis formation with remarkable therapeutic benefit in mouse models of prostate cancer. These articles emphasize the importance of specificity at the level of translational control downstream of oncogenic pathways, such as PTEN and mTOR, and represent valuable tools for the rational development of new cancer therapeutics.Swartling FJ, Savov V, Persson AI, Chen J, Hackett CS, Northcott PA, et al. Distinct neural stem cell populations give rise to disparate brain tumors in response to N-MYC. Cancer Cell 2012;21:601–13.The proto-oncogene MYCN regulates proliferation and differentiation in the normal brain. Misexpression of MYCN occurs commonly in childhood medulloblastoma and at a more limited frequency in adult brain tumors, including the malignant glioma. To explore a role for MYCN in these tumors, Swartling and colleagues transduced wild-type or mutationally stabilized murine N-mycT58A into primary neural stem cells (NSC) from perinatal murine forebrain and hindbrain (cerebellum and brainstem). Orthotopic injection of wild-type N-myc NSCs was insufficient for tumor formation. In contrast, NSCs from all 3 brain regions, in response to mutationally stabilized N-myc, formed tumors after orthotopic transplantation. NSCs derived from either cerebellum or brainstem generated medulloblastoma, whereas forebrain NSCs generated primitive-appearing diffuse glioma. The transcription factor Sox9 marked Sonic Hedgehog (SHH) dependence in human medulloblastoma and was a critical regulator of differentiation and self-renewal in NSCs and in tumor initiating cells. While mutationally stabilized N-myc initiated SHH-dependent tumors from both forebrain and embryonic cerebellar NSCs, mutationally stabilized N-myc drove SHH-independent medulloblastoma from postnatal cerebellar NSCs and from prenatal brainstem NSCs. These results demonstrate regional and age-dependent differences in the response of NSCs to a common oncogenic signal and inform the diversity of human brain tumors.Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.